Among a wide variety of protective relay devices used with power transmission lines, distance relays respond to faults which cause a less than normal impedance condition on the line, referred to as an under-impedance condition. A distance relay capable of accomplishing such protection includes one or more mho elements, each mho element having a circular impedance plane characteristic, such as shown in FIG. 1. FIG. 1 shows resistance R along one axis and reactance Z along the other axis. Z.sub.R at angle .theta. is the impedance setting of the particular mho element and defines the boundary or "reach" of the circular impedance plane characteristic shown in FIG. 1.
In FIG. 2, which is in the voltage plane, a voltage Z.sub.R.I, which is the product of the measured value of current on the transmission line at the relay and the relay impedance Z.sub.R, is shown, as is the measured value of voltage V on the transmission line at the relay. I and V are the instantaneous values of current and voltage on the transmission line at the relay, taken at the same time. The difference voltage dV between the measured voltage V and the calculated voltage Z.sub.R.I is also shown in FIG. 2. The three specific phasor quantities V, Z.sub.R.I and dV form a triangle, as shown. Angle "a", which is the angle between the voltage phasors V and dV at point P on the mho circle is 90.degree.. A distance relay is designed to discriminate between transmission line impedances which are larger or smaller than its characteristic impedance Z.sub.R, on the basis that a smaller impedance will result in a mho circle of smaller diameter, which results in an angle greater than 90.degree. between the V and dV phasors, while a larger impedance will cause the angle between the V and dV phasors to be less than 90.degree.. As shown in FIG. 3, this discrimination is accomplished by a 90.degree. phase comparator 10, which compares the measured instantaneous voltage V from the transmission line on input line 12 with the calculated value dV on input line 14. The output of the phase comparator 10 on line 16 provides an indication of whether the impedance condition of the transmission line is normal or less than normal, which is indicative of a possible transmission line fault. A great enough under-impedance condition will result in the relay producing a control signal to trip a circuit breaker for the particular transmission line in question, thus protecting the line.
Different types of phase comparators have been used in the manner described above. These include induction cylinder units, Hall Effect devices, and thermal and solid state networks. More recently, a system involving coincidence timing of sinusoidal inputs, involving the phase relationship between the input signals, has been used. Also, computers have been used to provide calculated simulations of mechanical and electro-mechanical devices. For instance, for an induction cylinder type of phase comparator, the difference between the input signals produces a mechanical torque which increases in magnitude in accordance with such difference, resulting in a contact closing when a predetermined under-impedance condition exists on the transmission line. In a computer, the mechanical action of the cylinder unit can be simulated through the multiplication of dV and a polarizing voltage. The resulting product can still be referred to conveniently as a "torque." The sign of the product indicates an under or over-impedance, i.e. whether the impedance is inside or outside the mho circle characteristic. The actual magnitudes of the torque values, however, were heretofore not considered to be useful.
In the protection of a typical transmission line which carries three phase AC power, a distance relay includes a plurality of individual mho elements, each having a specified impedance characteristic, to cover the various distance line fault possibilities. For instance, with three power phases A, B and C, a total of six mho elements will cover three phase-to-ground faults, i.e. A-G, B-G and C-G, as well as three phase-to-phase faults, i.e. A-B, B-C and C-A.
Although the above-described systems for determining distance faults, including those utilizing computer calculations instead of electrical-mechanical devices, provide to a significant extent accurate fault information, there are a number of fault situations which may not be accurately detected by the abovedescribed systems. For instance, when there is a fault close to the relay itself, the measured voltage V may approach zero, leading to inaccuracies in the resulting calculations. This is true in particular when the mho elements are self-polarized, i.e. the same voltage which is used as a component in determining the difference voltage dV is also used as the polarizing, i.e reference, voltage V for phase comparison.
To overcome this problem, polarizing voltages can be used which are based on memory, or which are based on a phase or phases free from faults FIG. 4 shows a mho element voltage plane characteristic using a polarizing voltage VP. The polarizing voltage increases the accuracy and fault-determining capacity of distance relays. However, such relays are still characterized by other significant problems. One problem concerns the response of more than one mho element to a particular fault, with resulting confusion as to the fault type. Providing reliable fault-type identification in single-pole tripping systems, for targeting and other applications, is also a significant issue in many systems. Attempting to satisfy all of the criteria leads to compromises in relay sensitivity, as well as increased cost when additional external logic must be utilized.
The present invention uses a particular form of polarizing voltage and a system for analyzing the calculated torque measurements from each mho element to provide a significant improvement in reliable fault information, including fault-type determination.